Check that the breather tube is not plugged or submerged under water first.
It is possible that the cleaning plug on the side of the pump head is not fully seated or slightly damaged. This will cause the wet/dry not to function properly.
Mine actually got damaged when I didn't insert it properly and crimped it when reassembling the pump head to the canister body after cleaning. I didn't have a spare so I used a rubber cork, trimmed to fit the hole. It now works perfectly well.
This is assuming that nothing else has changed on how you pack your filter with media, any add on equipment to the input/output lines or installation height changes made to the filter.
Keep us posted, Dee.